1. Field of the Invention
This invention relates to a diffraction grating for optical scanning in printers which print characters and designs onto a dry plate with a laser or other beam light source as well as bar-code readers.
2. Prior Art Statement
Rotary type diffraction gratings are used in beam-type printers and bar-code readers for guiding the light beam such that it scans across a surface to be scanned.
For example, a bar-code reader is shown in FIG. 1(a). A laser beam source 101 emits a laser beam 102 which passes through a diffraction grating 103 consisting a plurality of concentrically arranged grating elements provided on a disk scanner 110 and strike a bar code 105 placed upon a product 104. Light reflected from the bar code 105 is reflected by a mirror 106 into a photodetector 107, which reads the bar code 105 from the detected light. At this time, the disk scanner 110, radially provided with a plurality of diffraction gratings 103 as seen in FIG. 1(b), rotates about a shaft 108 so that the diffraction grating 103 moves accordingly, thus changing the position at which the laser beam 102a strikes the grating rings of the diffraction grating 103 and altering the direction in which the diffracted beam 102b travels, whereby the diffracted light scans across the entire width of the bar code 105.
The disk scanner described above is subject to the following requirements:
(i) The depth of the scanning spot must not change even if the scanning angle changes. Specifically, the surface over which the spot scans must be a plane.
(ii) The scanning spot must travel in a straight line upon the scanning surface.
(iii) The scanning spot must travel at a constant velocity.
Disk scanners have previously been made as holograms created from a plane wave and a divergent spherical wave, and have thus been known as hologram scanners, but the problem of (i) in particular could not be solved so they were not used in precision applications such as laser printers.
Specifically, as shown in FIG. 2, even though the spot of the diffracted beam 102b focuses exactly on the scanning surface 109 at point P.sub.1 in the central portion of the scanning angle .theta., at the two extremities of the scanning angle .theta., the spot will focus at points on the extensions of points P.sub.2 and P.sub.3 lying above the scanning surface 109, so accurate reading of the bar code 105 at points on the extension of points P.sub.2 and P.sub.3 is difficult. Therefore, it is desirable for points P.sub.2 and P.sub.3 of the beam 102b to also lie in the plane of the scanning surface 109.
To this end, a method known (Japanese Application public Disclosure No. 60-186805) in which a hologram is created by irradiation with a divergent spherical wave light from both sides and then another hologram is created from light from this hologram and divergent spherical wave light and then another hologram is created from light from this hologram and divergent spherical wave light, thus creating a hologram from a plurality of devergent spherical waves.
However, this method is complicated, and hologram are created successively from holograms, so it is difficult to create an accurate hologram.
The present invention came about in light of the above, and its object is to provide a diffraction grating for optical scanning in which the depth of the scanning spot of the light beam will not change even if the scanning angle changes, so therefore the scanning surface of the spot can be planar.